The present invention relates to a magnetic recording medium, and particularly to a magnetic recording medium improved to be suitable for high density magnetic recording by reducing noise generated from the magnetic recording medium and to a magnetic storage device using the same.
Studies have been made on a magnetic recording medium formed of a continuous magnetic thin film for realizing high density magnetic recording. Specifically, such a magnetic recording medium is prepared by a method wherein a thin film made of a ferromagnetic metal, Co or Co-based alloy is formed on a substrate made of a nonmagnetic material such as aluminum or glass coated with a plastic film or NiP film by radio frequency sputtering, ion beam sputtering, vacuum evaporation, electric plating or chemical plating. In the magnetic recording medium thus prepared, a microstructure of a magnetic thin film is closely related to magnetic properties. As a result, various attempts have been made to improve a magnetic layer constituting a magnetic recording medium for enhancing magnetic recording density and reproduced output.
For a longitudinal magnetic recording medium, it has been conceived that an easy magnetization axis thereof is desirable to be parallel to a substrate. On the other hand, various methods have been known to provide an underlayer between a substrate and a magnetic layer for ensuring longitudinal magnetic anisotropy. For example, U.S. Pat. No. 4,654,276 discloses a method in which a layer made of W, Mo, Nb or V is used as an underlayer for a Coxe2x80x94Pt magnetic layer. U.S. Pat. No. 4,652,499 discloses a method in which a Vxe2x80x94Cr or Fexe2x80x94Cr alloy material is used as an underlayer. Japanese Patent Laid-open No. Sho 63-106917 discloses a method in which a nonmagnetic layer made of Cr, Ho, Ti or Ta as an underlayer for a magnetic layer made of Co, Ni, Cr or Pt. U.S. Pat. No. 4,789,598 discloses a method in which Cr or a Crxe2x80x94V alloy is effective as an underlayer for a Coxe2x80x94Ptxe2x80x94Cr layer.
When a Co-based alloy magnetic layer is formed on a substrate through an underlayer made of Cr or a Cr alloy by sputtering, the underlayer is first oriented in (100) or (110). In this case, when the Co-based alloy magnetic layer is formed on the (100) orientated layer, the easy magnetization axis thereof is parallel to the substrate; while when the Co-based alloy magnetic layer is formed on the (110) oriented layer, the easy magnetization axis thereof is substantially in parallel to the substrate, more specifically, it is inclined at about 30xc2x0 relative to the surface of the substrate.
For improvement in an areal density of magnetic recording, it is required to reduce noise generated from a magnetic recording medium as well as to enhance resolution of magnetic recording. In particular, when a reproducing magnetic head of a magneto-resistance (MR) type being high in read-out sensitivity, it becomes important to reduce noise of a magnetic recording medium. The prior art magnetic recording medium of a type in which the easy magnetization axis thereof is oriented in the longitudinal direction has a disadvantage that it can improve resolution of magnetic recording; however, it has a difficulty in reducing noise thereof. In particular, for an areal recording density of magnetic recording increased to 1 Gb/in2 or more, the prior art magnetic recording medium is very difficult to reduce noise thereof.
An object of the present invention is to provide a magnetic recording medium suitable for high density magnetic recording and a magnetic storage device using the same.
The present inventors have experimentally studied magnetic recording media suitable for high density magnetic recording and found that the above-described object can be achieved by the following methods.
Specifically, it was revealed that a magnetic recording medium being low in degree of orientation or being isotropic (containing a perpendicular magnetization component) is superior in noise reduction to a magnetic recording medium with the easy magnetization axis thereof oriented in the longitudinal direction. Such a magnetic recording medium is required to satisfy the following requirement:
xe2x88x920.5xe2x89xa6{Hc(1)xe2x88x92Hc(P)}/Hc(1)xe2x89xa60.3
where Hc(1) is a coercivity measured in the longitudinal direction, and Hc(p) is a coercivity measured in the perpendicular direction.
In this requirement, to realize high density magnetic recording having an areal recording density of 1 Gb/in2 or more, the corecivity Hc(1) is required to be 2 kOe or more and a product of a remanent magnetization Br and a layer thickness xe2x80x9ctxe2x80x9d is required to be within the range of from 20 to 100 Gxc3x97xcexcm. When Hc(1) is less than 2 kOe or Brxc3x97t is more than 100 Gxc3x97xcexcm, resolution of magnetic recording fails to be enhanced. On the other hand, when Brxc3x97t is less than 20 Gxc3x97xcexcm, a sufficient signal output cannot be obtained upon reproduction of a recording signal by the magnetic head, the magnetic recording medium is difficult to be operated as a magnetic storage device.
To obtain a magnetic recording medium capable of satisfying the above-described requirement, the magnetic recording medium is required to ensure a high coercivity Hc(1) while thinning the thickness of a magnetic layer to 20 nm or less. In general, for a magnetic layer having a thickness of 20 nm or less, it is difficult to ensure a high coercivity. Consequently, to ensure a high coercivity of a magnetic recording medium using a magnetic layer having a thickness of 20 nm or less, a magnetic anisotropy energy Ku of the magnetic layer is required to be 3xc3x97106 erg/cm3 or more.
To obtain a high coercivity Hc(1) in a magnetic recording medium using a magnetic layer being thin in thickness, it is effective that the magnetic layer is of a laminated structure. Specifically, in the case where the thickness of a magnetic layer is limited for reducing the value of Brxc3x97t to 100 Gxc3x97xcexcm or less, the coercivity Hc(1) can be increased using the magnetic layer of a laminated structure in which two kinds or more magnetic layers different in composition are directly laminated, as compared with a single magnetic layer. While being not clear, the reason for this is conceived that stress and strain are generated at each interface between the magnetic layers because of a slight difference in lattice constant therebetween, thus contributing to improvement in corecivity. In this case, nonmagnetic elements of alloy components constituting the magnetic layers are collected at the interface between the magnetic layers. As a result, magnetic coupling between a plurality of the magnetic layers is weakened, causing an effect in reducing noise generated from the magnetic recording medium.
To positively weaken magnetic coupling between a plurality of magnetic layers, it is effective to insert a nonmagnetic layer at each interface between two kinds or more of the magnetic layers different in composition.
Another method may be also adopted to form a nonmagnetic material between crystal grains of a magnetic thin layer, wherein a magnetic layer is formed by sputtering, using an alloy target made of a Coxe2x80x94Cr, Coxe2x80x94Pt, Coxe2x80x94Crxe2x80x94Ta, or Coxe2x80x94Crxe2x80x94Pt alloy placed with pellets of a nonmagnetic material such as SiO2, ZrO2, TiB2, ZrB2, MoSi2, LaB6, SiC, B4C, or B6Si. In this method, an average grain diameter of magnetic crystals constituting the magnetic thin layer becomes smaller and also a thin layer made of nonmagnetic material is interposed between the crystal grains of the magnetic thin layer. In the magnetic recording medium having such a structure, the magnetic coupling force between magnetic crystal grains can be reduced, and thereby noise of the medium can be reduced. To realize a high density magnetic recording having an areal recording density of 1 Gb/in2 or more, the average grain diameter of magnetic crystals of a magnetic layer is desirable to be within the range of from 5 to 15 nm.
As a magnetic head in combination with such a magnetic recording medium, a composite head of a thin film ring head for recording and a magneto-resistance effect (MR) head being high in reproduction sensitivity for reproduction is desirable. To realize a high density magnetic recording having an areal recording density of 1 Gb/in2 or more, a linear recording density of 100 kFCI or more is generally required, and in this case, a distance between the magnetic head and the surface of a magnetic film of a magnetic recording medium is required to be 0.08 xcexcm or less. The smaller the distance, the better the high density recording. However, when the distance is 0.02 xcexcm or less, the thickness of a protective layer and a lubricant layer provided on the surface of the magnetic layer becomes significantly thin. This is poor in usability in terms of tribological reliability.
To realize a high density magnetic recording having an areal recording density of 1 Gb/in2 or more, the track width of a magnetic head is also required to be made smaller. For a linear recording density of 100 kFCI, the track density must be 10 kTPI or more for ensuring the areal recording density of 1 Gb/in2 or more. In this case, the track pitch becomes about 2.5 xcexcm or less. When a guard band of 0.5 xcexcm is set between recording tracks, the track width of a magnetic head must be 2 xcexcm or less. On the other hand, a magnetic head having a track width being 0.3 xcexcm or less is difficult to be practically prepared. The track width of a magnetic head in combination with the magnetic recording medium is thus within the range of from 0.3 to 2.0 xcexcm. To realize an areal density of 4 Gb/in2 or more, a giant magneto-resistance effect (G-MR) head being higher in sensitivity than the MR head is desirable to be used as the reproducing head.
According to the present invention, there can be provided a magnetic recording medium suitable for high density magnetic recording by reducing noise generated therefrom and suppressing an error rate thereof, and thereby a magnetic storage device having an areal recording density of 1 Gb/in2 or more can be realized.